Four logic probe circuit diagrams

Simple logic probe

When the probe is not touching any circuit, the circuit does not consume current. The reason is that the voltage from the green LED to the base-emitter junction of BC557, then to the base-emitter junction of BC547 and the red LED is about: 2.1V+0.6V+1.7V+0.6V=5v, which is greater than the power supply voltage.

When the circuit detects a low level, BC557 turns on the green LED. When a high level (above 2.3V) is detected, the red LED turns on.

Simple logic pulse probe

The circuit has a surprisingly high impedance, the probe is not touching any circuit and no current is consumed when the input is present.

Keep the probe away from stray signals (especially power supply hum) or the orange LED will light up.

Because the PN junction voltage drop of the diode and transistor will reduce the voltage on the emitter of the fourth transistor to less than 1v, an additional 1.5v battery is required to light up the orange LED.

Logic Pulse Probe

This circuit has the advantage of being able to detect high and low of the same logic level.

Continuity test

This circuit has the advantage of providing a beeping sound when a logic signal is detected, it cannot detect a signal less than the voltage drop across the diode. When testing a logic circuit, it is a rapid beeping sound that you can listen to, which is much faster than testing with a multimeter.